This invention relates to a method of toughening a glass sheet, for example for use as an automobile window, by heating it to a temperature above the strain point and quenching the heated glass sheet by blowing a cooling medium such as compressed air against both sides of the glass sheet.
Such quenching of the glass sheet results in the production of permanent compressive stresses in the surface layers of the glass sheet, with compensating tensile stresses in the center of the glass thickness. When the toughened glass sheet is fractured by localized impact, cracks tend to extend radially from the point of impact and branching of each crack occurs at indefinite intervals until the glass sheet breaks up into small particles of glass.
In many countries there are official regulations specifying the manner of fragmentation of toughened glass sheets for use as windows of motor vehicles. Such regulations commonly require that the fractured glass sheet should not contain dangerously elongated particles. For example, British Standard BS 5282 basically prohibits the presence of particles longer than 60 mm in which the length is not less than four times the width. Besides, BS 5282 specifies that the number of particles included in any 50 mm.times.50 mm square traced on the glass sheet should be within a limited range and further specifies a maximum permissible area of each particle. In the recent automobile industry there is a trend to reduce the thickness of the glass of windows to thereby aid the reduction of the gross weight of the car. However, for glass sheets of about 3.5 mm or less thickness it becomes difficult to toughen them so as to meet the official regulations, and the difficulty becomes serious for glass sheets of relatively large sizes particularly in respect of the exclusion of elongated particles.
In conventional methods of toughening a glass sheet by quenching, it is usual to use a pair of opposite quenching headers each having a faceplate which is parallel to and slightly wider than the glass sheet and in which a series of quenching nozzles are plugged to jet out compressed air perpendicularly against the surface of the glass sheet. Usually the nozzles on each faceplate are arranged in parallel rows in the pattern of a square lattice or in alternately staggered rows.
British Patent No. 1,512,163, which is concerned with toughening of glass sheets of 2.5-3.5 mm thickness so as to meet the requirements of BS 5282 by using a quenching apparatus of the above described type, proposes to differentially quench the glass sheet so as to produce in the quenched glass sheet a distribution of highly toughened regions, less toughened regions and intermediately toughened regions which are adjacent to one another by oscillating the whole quenching nozzles parallel to the glass sheet either reciprocatively over a distance corresponding to the spacing between the adjacent two nozzles or circularly with an adequate radius. However, this method does not seem to be convenient for industrial practice. Furthermore, the arrangement of the quenching nozzles in the pattern of a square lattice or staggered rows itself does not seem to be quite reasonable when considered from the mechanism of the propagation of cracks at the fracture of the toughened glass sheet. It is known that the cracks tend to run substantially perpendicular to the direction of principal stress in the glass. However, in a glass sheet toughened by using quenching nozzles arranged in a lattice pattern it is likely that principal stresses become too large in some specific regions so that, when the glass sheet is fractured, long and straight cracks that will possibly produce impermissibly elongated particles tend to develop in not small numbers. Besides, the difference between the maximum particle count and the minimum particle count in unit area of the fractured glass sheet is liable to become too large, and it is not rarely that some particles have areas larger than the maximum permissible area. Even though certain compensating measures such as the oscillation of the quenching nozzles described in GB 1,512,163 and controlled transfer of the glass sheet transversely of quenching jets according to separate proposals will be effective to some extent, the intrinsic disadvantage of lattice pattern arrangement of the quenching nozzles remains undissolved. Furthermore, such compensating measures require very careful consideration of various factors.
Japanese patent application primary publication No. 57-129835 (1982) proposes to arrange a set of quenching nozzles on the contours of a number of successive hexagons drawn in a plane parallel to the glass sheet to be toughened. In our view, however, this technique is not fully effective in desirably toughening relatively thin glass sheets.